Traditionally flexible pipe is utilized to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater, say 1000 meters or more, or shallower water, say 250 meters to 1000 meters) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 meters. Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body often generally includes metallic and polymer layers.
In many known flexible pipe designs the pipe body includes one or more tensile armour layers. The primary loading on such a layer is tension. In high pressure applications, such as in deep and ultra deep water environments (deep water being considered as less than 3,500 feet (1,005.84 meters) and ultra deep water as greater than 3,500 feet), the tensile armour layer experiences high tension loads from a combination of the internal pressure end cap load and the self-supported weight of the flexible pipe. This can cause failure in the flexible pipe since such conditions are experienced over prolonged periods of time.
One technique which has been attempted in the past to in some way alleviate the above-mentioned problem is the addition of buoyancy aids at predetermined locations along the length of a riser. The buoyancy aids provide an upwards lift to counteract the weight of the riser, effectively taking a portion of the weight of the riser, at various points along its length. Employment of buoyancy aids involves a relatively lower installation cost compared to some other configurations, such as a mid-water arch structure, and also allows a relatively faster installation time.
An example of a known riser configuration using buoyancy aids to support the riser is a stepped riser configuration 100, such as disclosed in WO2007/125276 and shown in FIG. 1, in which buoyancy aids 101 are provided at discrete locations along a flexible pipe 103. Alternatively the buoyancy aids can be provided in groups (i.e. 2 or more buoyancy aids) at discrete locations along a flexible pipe. The riser is suitable for transporting production fluid, e.g. oil and/or gas and/or water from a subsea location to a floating facility 105 such as a platform or buoy or ship. A further example of a known riser configuration using buoyancy aids is a lazy wave configuration 200 shown in FIG. 2, in which buoyancy aids 201 are provided at points (positions) along a flexible pipe 203 so as to provide a ‘hog bend’ in the riser. The lazy wave configuration is often preferred for shallow water applications. In such applications there are often significant dynamic motions in the flexible pipe as a result of vessel or platform movement, currents and sea states. These can lead to large stress variations in layers of the pipe body and rapidly accumulate fatigue damage as a result. This can be exacerbated by the presence of buoyancy which increases the incident cross section of the flexible pipe configuration on which the currents will act.
WO2007/125276 discloses a flexible pipe including rigid buoyancy supports at one or more points along a riser assembly. The rigid buoyancy support provides a rigid surface to affix buoyancy aids to the flexible pipe, thereby avoiding crushing of the flexible pipe due to compression loads being exerted as the buoyancy aid is attached.
Other riser configurations may require the addition of ballast weight to a flexible pipe to decrease the buoyancy of the pipe at one or more positions to suit a particular marine environment or production fluid extraction set up.
A problem that is experienced with flexible pipes, in particular when used in a riser configuration, is that water flowing past the pipe causes vortex shedding. Vortices are created on the downstream side of the pipe, which are shed from alternate sides and can give rise to pressure variations and result in excessive motions of the pipe. The term vortex induced vibration (VIV) has been coined in the art to describe the phenomenon which results in such problems.
The current flows, if strong enough and sufficiently uniform or consistent, can result in vortex shedding which can excite the natural resonant/oscillatory frequency of the pipeline. The kinetic energy delivered to the pipeline can in these circumstances be significant enough to cause fatigue failure of the pipe or damage to surrounding or connecting structures as some of the motion and energy is transmitted to them from the vibrating pipe.
Buoyancy sections of a riser can be subject to these vortex shedding forces. For a buoyancy aid in use underwater, which may have a cylindrical shape, water flow passing around the buoyancy aid can form a boundary layer. Resultant vortices change the pressure distribution along the surface of the buoyancy aid and resultant net forces may cause the buoyancy aid to move transversely. If the frequency of VIV is close to the resonant frequency of the pipeline in its installed configuration, including the buoyancy aid, the riser will experience larger oscillations of movement and this may lead to failure of the riser system.
As used herein, the term “buoyancy compensating element” is used to encompass both buoyancy aids for increasing buoyancy and ballast weights for decreasing buoyancy.